High speed differential signal traces are widely used in a variety of products such as servers or storage products. Many servers and storage products include a chassis that supports and connects different printed circuit boards having various electronic devices mounted thereon. For example, a printed circuit board may have one or more integrated circuit chips mounted thereon, alone or in combination with analog devices or other devices. The printed circuit boards include various signal traces to provide signals to the devices on the boards. Signal traces generally are arranged in differential trace pairs for a particular signal.
As printed circuit boards become more and more complex with the inclusion of more components, the routing for connections between components requires longer signal path lengths. FIG. 1 is an example of a circuit board 10 with a prior art signal trace 12 that serves as a signal channel connecting a transmitter 14 to a receiver 16. As the trace 12 gets longer due to spacing between the transmitter 14 and the receiver 16, it becomes more lossy, which degrades signal modulation of data. When signals are transmitted through a lossy channel, high and low frequency portions of the signal will have different attenuation. The longer the channel, the larger the deviation in attenuation between the high and low frequency portions of the signal. This deviation in attenuation can cause distortion when the receiver 16 samples and recovers bit information from input signals originating from the transmitter 14 and received through the signal trace 12.
This effect may be seen in the waveforms of a signal received from the signal trace 12. FIG. 2A is a plot of signal amplitude as a function of time, where line 20 shows a signal from the transmitter 14 being transmitted over the signal trace 12 in FIG. 1. FIG. 2B is also a plot of signal amplitude as a function of time, where lines 22 and 24 show signals received by the receiver 16 in FIG. 1 for different lengths of signal trace 12. In particular, line 22 shows the signal at the receiver 16 when the signal trace 12 has a length of 6000 mil. In contrast, a line 24 shows the signal at the receiver 16 when the signal trace has a length of 12000 mil. FIG. 2C is a plot of attenuation as a function of frequency for different lengths of signal trace 12. In particular, line 32 shows the attenuation for the shorter channel length (6000 mil), and line 34 shows the attenuation for the longer channel length (12000 mil). As may be seen in FIG. 2C, the attenuation increases as a function of frequency for both lengths. However, the amount of attenuation increases as the length of the trace increases. This is shown by the greater attenuation in line 34 versus line 32. Consequently, this indicates that the distortion in the signal trace increases as the length of the signal trace increases.
For such longer path applications, active components like a re-driver are introduced and used in recent designs in order to allow sufficient signal transmission. For example, in a typical server, 20″ data paths are used. The recent Peripheral Component Interconnect Express (PCIe) 4 standard requires a re-driver to insure signal integrity over the length of the data path in the server. A re-driver is a signal amplifier that receives a signal at a point of the channel and amplifies the signal to reduce attenuation. Re-drivers therefore are currently required to deploy PCIe 4 type components in servers. However, a re-driver component increases system power requirements and design complexity. It is therefore desirable to simplify design complexity and reduce system power by minimizing the number of re-drivers. However, this inhibits flexibility of design, since longer traces cannot be used. Thus connected components must be placed in relative proximity to each other.
There is therefore a need for a passive component to reduce deviation between high and low frequency attenuation to allow longer signal path applications. There is a further need for a component that allows maintaining the same system power over the length of a return path in a differential signal trace pair. There is a further need for a passive component that does not require additional energy for operation.